Cellular systems are evolving toward using Orthogonal Frequency Division Multiplexing (“OFDM”), where the data for one user is modulated across many subcarriers and then modulated onto a single main carrier. Other sub-carriers, called pilots, may contain no underlying data. In practice, data in a pilot is set to all ones or all zeros. Pilots are generally used to help acquire and synchronize a signal. Due to oscillator instabilities, however, the frequency may drift from the intended value, resulting in a Carrier Frequency Offset (“CFO”). In demodulating OFDM signals, the CFO should be accurately estimated for good performance. Typically, if there are multiple users in a cell, the users in the cell each occupy different carrier frequencies, and hence different bands. If the CFO for a given user cannot be estimated accurately, the performance of OFDM systems can degrade significantly.
Co-channel interference in cellular telecommunications systems is a significant problem for signal detection and demodulation on both terrestrial receivers and space-based platforms, such as satellites. Existing methods for CFO estimation based on the Multiple Signal Classification (“MUSIC”) algorithm, Kalman filtering, or other techniques such as Maximum Likelihood Sequence Estimation (“MLSE”) only apply to a single user in a given frequency band, or to a multi-user system where the multiple users do not overlap in frequency (i.e., they are not co-channel users). Hence, conventional techniques do not address the co-channel interference problem where two different users' signals overlap in time and frequency and both CFOs must be estimated.
In such a case, the users overlap in the same time and in the same frequency band, but with slightly offset carrier frequencies. Typically, in such a scenario, the signal for the lower powered user would be ignored (i.e., dropped). This would force the user to initiate the call again in order to continue the conversation. This co-channel interference scenario is common in real systems, but is not addressed conventionally. Accordingly, a method of effectively handling co-channel interference may be beneficial.